Various rotary valves, such as ball valves have been used as control valves for fine control of fluid flow characteristics on a continuing basis as distinguished from conventional on and off operation. However, certain of such valves are difficult to turn and require a large, high torque actuator, maximizing cost and bulk. Costs have been reduced in some cases by minimizing the weight of a valve closure member, as by forming the plug as merely a segment of a sphere. However, the force of the flow stream acting against the plug still imposes certain torque loads which makes the desired close control difficult.